Superalloys are metallic materials having useful properties at temperatures on the order of 1000.degree. F. and above and find application in gas turbine engines. One large class of superalloys are nickel base alloys which are strengthened by the gamma prime phase (based on Ni.sub.3 Al) in particulate form. Such superalloys generally display a reduction in the gamma prime content with increasing temperature and at a particular temperature (the gamma prime solvus temperature), the gamma prime phase is completely dissolved into solid solution and no gamma prime particles are present. While some superalloys melt before the gamma prime phase is dissolved, many alloys of commercial interest exhibit a gamma prime solvus temperature.
Initially superalloys were employed in equiaxed polycrystalline form. Each article was composed of many individual (essentially equiaxed) crystals which are randomly oriented with respect to each other. Advances in the understanding of the mechanical behavior of materials led to the development of columnar grain and single crystal technology. Single crystals are now used in some gas turbine engines particularly as blades since it has been found that at elevated temperatures the properties of single crystals are substantially superior to those of the prior art polycrystalline material.
It is also known that although the gamma prime phase when present effectively inhibits recrystallization, if a superalloy is deformed more than about 2% and then heated to a temperature above the gamma prime solvus temperature, the material will recrystallize and new grains will form in the area which has been deformed (the actual behavior is more complex than this, depending in part upon the deformation temperature). Formation of such new grains is undesirable in a single crystal since the grain boundaries which separate the new grains from the original single crystals will be areas of weakness and will reduce the effectiveness of the article in service. There are also situations in polycrystalline materials, where a particular condition of grain size and/or preferred orientation must be maintained, where recrystallization will adversely affect the material properties and should be avoided.
It is known to form a complex superalloy article by diffusion bonding two or more components together. It is also known that single crystal components can be bonded in such a way that the boundary between the single crystals is innocuous, or even nonexistent if the crystals are properly oriented--see U.S. Pat. No. 4,033,041. However, in diffusion bonding such articles localized deformations in excess of 2% can occur resulting in recrystallization during subsequent heat treatment.